Many solid state lighting systems SSL, which are targeted at high-end applications, comprise an optical and/or thermal feedback loop for ensuring color consistency of the incorporated light emitting devices over operating conditions. The light emitting diodes (LEDs) may be disposed underneath an optical systems, which is provided for mixing the colors of the light emitting diodes. An optical sensor measures color and/or intensity of the light (or a sampled representative portion) and provides a feedback signal in accordance with the measured signals to a controller of the lighting system. Such a controller may be an ASSP controller. Placed on user defined color settings and brightness control settings, the ASSP controller provides a control signal, which modulates the LED voltage/current and hence the light. The control method can be pulse width modulation (PWM), pulse code modulation or amplitude modulation (PCM), stochastic pulse density modulation (SPDM) or any other LED control methodology known to the person skilled in the art. In one implementation, the feedback loop requires logic circuitry as well as control mechanisms. This logic task is accomplished by a microcontroller or a state machine, which is to be added to the SSL system to calculate the new settings of current and modulation of the LED according to the feedback results. The control loop maintains basically fixed light output and color setting over variations of time, temperature, and drive conditions. Sensors to be used as color sensing means may be thermal sensors, flux sensors, or color sensors. Typical applications to be implemented by those systems can contain a plurality of light engines featuring light emitting semiconductor devices of several thousands of lumen.
The term light engine is used for defining a device which emits radiation in any region or combination of regions of the electromagnetic spectrum for example, the visible region, infrared and/or ultraviolet region, when activated by applying a potential difference across it or passing a current through it, for example. Therefore a light engine can have monochromatic, polychromatic or broadband spectral emission characteristics. The light engine can in certain implementations also be controlled via a control interface.
FIG. 1 shows a simplified block diagram of a lighting system according to the prior art. The lighting system comprises a AC/DC converter 107, a bus microcontroller 108, a first light engine 101 coupled to several strings of light emitting diodes LEDs and a first optical sensor 105. Each of the strings can be comprised of a different LED colors (for instance, red, blue, green or white) or a combination of colors (e.g. red plus amber). The lighting system furthermore comprises a second light engine 103 which is coupled to several strings of light emitting devices LEDs and a second optical sensor 106. Two light engines 101 and 103 are coupled to several strings of light emitting devices, in this case light emitting diodes. The devices can also be other semiconductor based light emitting devices known by persons skilled in the art such as lasers, OLEDs.
In the example of FIG. 1, the light emitting diodes (LEDs) include a string of red LEDs indicated by R, a string of green LEDs indicated by G, and a string of blue LEDs indicated by B. Further, there is a single LED of amber indicated by A. The light engines 101 and 103 can be realized by using integrated circuits which include the necessary driving and control circuitry in order to drive the light emitting diodes, i.e. the assembly of light emitting diodes in response to a signal provided by the respective optical sensors 105 and 106. In particular, one or a multiplicity of dedicated optical sensors 105 are coupled to a specific light engine 101, and another single or multiplicity of optical sensors 106 are coupled to another light engine 103. Each group of a light engine 101, 103, a group of light emitting diodes, and an optical sensor 105, 106 constitutes its own closed loop control circuit.
The AC/DC converter 107 converts an alternate current power supply of for example 230 volt into a DC voltage of for example 24 volt or the like. The light engines 101 and 102 typically both comprise a microcontroller or another analogue or digital control means 102 and 104, respectively. The controllers 102, 104 are integrated in the light engines in order to establish the necessary control mechanism of the LEDs. External control and adjustment of the lighting system can be supplied via a control bus 110, which is for example can be a PC bus, DMX bus, DALI bus, CAN bus or any other bus protocol known to persons skilled in the art. The luminaire bus microcontroller 108 receives external instructions such as brightness, color-point or light effects via another bus using typical communication protocols as DMX, DALI, or ZigBee. The typical features of this conventional architecture provide that every light engine comprises its own dedicated controller for controlling the LED coupled to the light engine. Accordingly, controllers are provided at both, the luminaire and the light engine level. Further, every light engine has its own optical feedback loop, with a single flux or color sensor per system.
U.S. Pat. No. 6,507,159 B2 relates to a system for RGB based luminary. A plurality of photo diodes are used as color sensors to detect light from light emitting diodes. The measurements of the photo diodes are forwarded to a controller which is adapted to control the light emitting diodes such that a feedback loop is established.
U.S. Pat. No. 6,495,964 B1 concerns a luminaire system with red, green and blue light emitting diodes and a photodiode arrangement for measuring the light from the RGB light emitting diodes LED. The light of the RGB LED is measured in each color separately in a sequence of time pulses. The measurements of the photodiodes are forwarded to a controller which is controlling the RGB LED to provide a feedback loop.
Therefore, a basic disadvantage of the prior art consists in the considerably high number of controllers and sensors necessary to establish feedback control of the prior art systems.